The present invention relates generally to lifting cranes used in the construction industry, and more particularly to mobile cranes having main and auxiliary counterweights.
Most crane companies typically have one or more general, all-purpose truck cranes or crawler cranes which are usable in the majority of lifting and moving projects encountered in a typical construction project. Owing to physical and legal requirements concerning length, height, weight, etc., such cranes usually must be partially disassembled in order to be moved between job sites.
Cranes of this type are disclosed, for example, in U.S. Pat. No. 5,035,337 to Juergens, U.S. Pat. No. 5,586,267 to Landry, and U.S. Pat. No. 5,222,613 to McGhie.
The above mentioned cranes typically include: a wheeled or crawler mounted base; a platform pivotally mounted on the base for rotation about a vertical axis; a boom pivotally mounted on a forward end of the platform for rotation about a horizontal axis, the boom having an end projecting upwardly and forwardly relative to the platform; a load line having two ends, with one end connected to a winch mounted on the platform, the other end passing over the end of the boom and connected to a load; and a counterweight attached to the rearward end of platform to counteract the tilting effect or moment arm created by the boom and a weight to be lifted.
Cranes of the type described above may also be provided with an auxiliary counterweight assembly to offset tilting effects imposed by extremely large loads. For example, the above mentioned Juergens U.S. Pat. No. 5,035,337 discloses a crane having an auxiliary counterweight which, along with a main counterweight, is attached to a spar which extends rearwardly from a platform. A first end of the spar is pivotally attached to the platform while a second end is attached to a boom via a pendant. Note that the auxiliary counterweight is normally ground supported, and for that reason, the second end of the spar must extend beyond the platform and base of the crane. In response to lifting a heavy load, a load line exerts a bending moment on the boom which, by virtue of the pendent, exerts a bending moment on the spar. The spar, which has a limited or lost range of motion, reacts by lifting both the main counterweight and the auxiliary counterweight to counteract the heavy load. The problem with cranes of this type is that main counterweight, the auxiliary counterweight and the spar combine to create a large tail swing which prevents the crane from operating in restricted spaces without interference from obstacles adjacent the rear end of its wheeled base. Even when the auxiliary counterweight is removed, the spar and its associated main counterweight still project rearwardly beyond the platform and the base of the crane to create a large tail swing. Additionally, the auxiliary counterweight, the main counterweight and the spar must be assembled and disassembled each time the crane is moved. This means that there are considerable periods of time when the crane is not generating income.
The crane of Landry U.S. Pat. No. 5,586,667 is similar to Juergens except for the design of the spar. In Landry, the spar is separated into two parts, a main spar and a spar tip. The main spar has first and second ends with the first end rigidly attached to the platform and the second end providing a pivot attachment point for the spar tip. The spar tip also has first and second ends, with the first end pivotally attached to the second end of the main spar. The second end of the spar tip functions as an attachment point for an auxiliary counterweight and as an attachment to the boom end of the crane. Motion between the main spar and the spar tip is limited by a lost motion connection. As in the patent to Juergens, Landry attaches the main and auxiliary counterweights to the spar. And, as in Juergens, Landry has the same disadvantages of a large tail swing and extended assembly/disassembly time.
In McGhie U.S. Pat. No. 5,222,613, an auxiliary counterweight system is removably connected to a platform of a crane via pivot pins and a boom hoist rope. The system comprises a counterweight carriage, a counterweight, and a coupling means. The coupling means is in the form of a yoke and includes yoke extensions with pivot pin apertures and a sheave bracket mount with sheaves rotatably mounted thereto. To attach the system, pivot pins are inserted into the pivot pin apertures and apertured ears which project from the crane and which are aligned therewith. Then the boom hoist rope is re-reeved about the sheaves in the sheave bracket mount. This crane is capable of operating without the auxiliary counterweight. However, in order to make such a conversion, the boom hoist rope must be disengaged from the sheave bracket mount and re-reeved about the platform sheaves, the pivot pins must be withdrawn from the pivot pin apertures and apertured ears of the crane, and the coupling means removed. This procedure takes time. If the coupling means is not removed, tail swing remains unnecessarily large and potentially dangerous.
For the foregoing reasons, there is a need for a crane that can operate in relatively restricted space without interference from obstacles adjacent the rear end of its base, and whose lift capacity can be easily and rapidly changed without disassembly in response to varying conditions.